The present invention concerns improvements in selective catalytic reduction of NOx in waste gas streams such as diesel engine exhaust or other lean exhaust gases such as from gasoline direct injection (GDI).
EP 0 758 713 (Toyota) describes a method for purifying the exhaust gas of a diesel engine which uses a catalyst to convert NO in the exhaust gas to NO2, there after trapping particulate in a filter and oxidising the particulate by reaction with the NO2. The exhaust gas is thereafter fed to a NOx absorbent or, in one embodiment, to a NOx conversion catalyst. Unburnt hydrocarbons and CO in the exhaust gas are trapped in a zeolite and released to react with NOx on the NOx conversion catalyst.
The technique named SCR (Selective Catalytic Reduction) is well established for industrial plant combustion gases, and may be broadly described as passing a hot exhaust gas over a catalyst in the presence of a nitrogenous reductant, especially ammonia or urea. This is effective to reduce the NOx content of the exhaust gases by about 20-25% at about 250xc2x0 C., or possibly rather higher using platinum catalyst, although platinum catalysts tend to oxidise NH3 to NOx during higher temperature operation. We believe that SCR systems have been proposed for NOx reduction for vehicle engine exhausts, especially large or heavy duty diesel engines, but this does require on-board storage of such reductants, and is not believed to have met with commercial acceptability at this time.
We believe that if there could be a significant improvement in performance of SCR systems, they would find wider usage and may be introduced into vehicular applications. It is an aim of the present invention to improve significantly the conversion of NOx in a SCR system, and to improve the control of other pollutants using a SCR system.
Accordingly, the present invention provides an improved SCR catalyst system for treating combustion exhaust gas containing NO and particulates, comprising in combination and in order, an oxidation catalyst effective to convert NO to NO2 and enhance NO2 content of the exhaust gas, a particulate filter, a source of reductant fluid, injection means for said reductant fluid located downstream of said particulate trap and an SCR catalyst.
The invention further provides an improved method of reducing pollutants, including particulates and NOx in gas streams, comprising passing such gas stream over an oxidation catalyst under conditions effective to convert at least a portion of NO in the gas stream to NO2, and enchance the NO2 content of the gas stream, removing at least a portion of said particulates in a particulate trap, reacting trapped particulate with NO2, adding reductant fluid to the gas stream to form a gas mixture downstream of said trap, and passing the gas mixture over an SCR catalyst.
Although the present invention provides, at least in its preferred embodiments, the opportunity to reduce very significantly the NOx emissions from the lean (high in oxygen) exhaust gases from diesel and similar engines, it is to be noted that the invention also permits very good reductions in the levels of other regulated pollutants, especially hydrocarbons and particulates.
The invention is believed to have particular application to the exhausts from heavy duty diesel engines, especially vehicle engines, eg truck or bus engines, but is not to be regarded as being limited thereto. Other applications might be LDD (light duty diesel), GDI, CNG (compressed natural gas) engines, ships or stationary sources. For simplicity, however, the majority of this description concerns such vehicle engines.
We have surprisingly found that a xe2x80x9cpre-oxidisingxe2x80x9d step, which is not generally considered necessary of the low content of CO and unburnt fuel in diesel exhausts, is particularly effective in increasing the conversion of NOx to N2 by the SCR system. We also believe that minimising the levels of hydrocarbons in the gases may assist in the conversion of NO to NO2. This may be achieved catalytically and/or by engine design or management. Desirably, the NO2/NO ratio is adjusted according to the present invention to the most beneficial such ratio for the particular SCR catalyst and CO and hydrocarbons are oxidized prior to the SCR catalyst. Thus, our preliminary results indicate hat for a transition metal/zeolite SCR catalyst it is desirable to convert all NO to NO2, whereas for a rare earth-based SCR catalyst, a high ratio is desirable providing there is some NO, and for other transition metal-based catalysts gas mixtures are notably better than either substantially only NO or NO2. Even more surprisingly, the incorporation of a particulate filter permits still higher conversions of NOx.
The oxidation catalyst may be any suitable catalyst, and is generally available to those skilled in art. For example, a Pt catalyst deposited upon a ceramic or metal through-flow honeycomb support is particularly suitable. Suitable catalysts are e.g. Pt/A12O3 catalysts, containing 1-150 g Pt/ft3 (0.035-5.3 g Pt/liter catalyst volume dependingbon the NO2/NO ratio require. Such catalysts may contain other components providing there is a beneficial effect or at least no significant adverse effect.
The source of reductant fluid conveniently uses existing technology to inject fluid into the gas stream. For example, in the tests for the present invention, a mass controller was used to control supply of compressed NH3, which was injected through an annular injector ring mounted in the exhaust pipe. The injector ring had a plurality of injection ports arranged around its periphery. A conventional diesel fuel injection system including pump and injector nozzle has been used to inject urea by the present applicants. A stream of compressed air was also injected around the nozzle; this provided good mixing and cooling.
The reductant fluid is suitably NH3, but other reductant fluids including urea, ammonium carbamate and hydrocarbons including diesel fuel may also be considered. Diesel fuel is, of course, carried on board a diesel-powered vehicle, but diesel fuel itself is a less selective reductant than NH3 and is presently not preferred.
Suitable SCR catalysts are available in the art and include Cu-based and vanadia-based catalysts. A preferred catalyst at present is a V2O5/WO3/TiO2 catalyst, supported on a honeycomb through-flow support. Although such a catalyst has shown good performance in the tests described hereafter and is commercially available, we have found that sustained high temperature operation can cause catalyst deactivation. Heavy duty diesel engines, which are almost exclusively charged, can produce exhaust gases at greater than 500xc2x0 C. under conditions of high load and/or high speed, and such temperatures are sufficient to cause catalyst deactivation. In one embodiment of the invention, therefore, cooling means is provided upstream of the SCR catalyst. Cooling means may suitably be activated by sensing high catalyst temperatures or by other, less direct, means, such as determining conditions likely to lead to high catalyst temperatures. Suitable cooling means include water injection upstream of the SCR catalyst, or air injection, for example utilising the engine turbocharger to provide a stream of fresh intake air by-passing the engine. We have observed a loss of activity of the catalyst, however, using water injection, and air injection by modifying the turbocharger leads to higher space velocity over the catalyst which tends to reduce NOx conversion. Preferably, the preferred SCR catalyst is maintained at a temperature from 160xc2x0 C. to 450xc2x0 C.
We believe that in its presently preferred embodiments, the present invention may depend upon an incomplete conversion of NO to NO2. Desirably, therefore, the oxidation catalyst, or the oxidation catalyst together with the particulate trap if used, yields a gas stream entering the SCR catalyst having a ratio of NO to NO2 of from about 4:1 to about 1:3 by vol, for the commercial vanadia-type catalyst. As mentioned above, other SCR catalysts perform better with different NO/NO2 ratios. We do not believe that it has previously been suggested to adjust the NO/NO2 ratio in order to improve NOx reduction.